Blender is saving as a blend1 file, not blend
That's Blender's file versioning system. A very subtle but quite helpful feature. When you open the settings panel, under File, you can specify how many previous versions of a file you want to keep around on your computer. 

So everytime you hit save, Blender maintains a stack of previous versions of the same file so you have some backup of your work and don't overwrite your progress when saving.



In the above screenshot, the last 5 versions of one .blend file are saved. All suffixed individually (.blend1, .blend2 and so on). 

You can enable showing file versions in the open file dialog if you want to pick a specific version from your save history (red arrow). And also you can toggle showing normal .blend files in that dialog (green arrow) which is what your student probably did.



Hope this helps!